Conductors (i.e. cables) are one method commonly used to convey electronic video signals from a source device (e.g., a video camera or a DVD player) to a destination device (e.g., a video display screen). Two types of cable commonly used for video transmission are coaxial cable and twisted pair cable. It is desirable for the video signal at the destination device to correspond accurately to the original video signal transmitted by the source device. “Insertion loss” is a term used to describe signal degradation that occurs when a video or other signal is transmitted over a transmission medium such as a cable.
Typically, insertion loss is a function of the cable length: longer length transmission cables will exhibit greater loss than shorter length cables. Coaxial cables typically exhibit less insertion loss than twisted pair cables. However, coaxial cables are more expensive and difficult to install than twisted pair cables. Twisted pair cables typically are manufactured as bundles of several twisted pairs. For example, a common form of twisted pair cable known as “Category 5” or “CAT5” cable comprises four separate twisted pairs encased in a single cable. CAT5 cable is typically terminated with an eight-pin RJ45 connector.
Insertion loss is typically caused by the physical characteristics of the transmission cable. Insertion loss includes resistive losses (also sometimes referred to as DC losses) as well as inductive, capacitive and skin effect losses (also sometimes referred to as AC losses). The AC insertion loss exhibited by a cable is frequency dependent. For example, the insertion loss for a 1500 foot length of CAT5 cable as a function of frequency is shown in FIG. 11. In the example of FIG. 11, the insertion loss generally increases with increasing frequency, with the insertion loss for high frequency signals being significantly greater (−70 dB at 50 MHz for a 1500 feet CAT-5 cable) than the DC insertion loss of 2.6 dB for 1500 Feet (e.g. the loss at a frequency value of zero).
Video signals come in a variety of formats. Examples are Composite Video, S-Video, and YUV. Each format uses a color model for representing color information and a signal specification defining characteristics of the signals used to transmit the video information. For example, the “RGB” color model divides a color into red (R), green (G) and blue (B) components and transmits a separate signal for each color component.
In addition to color information, the video signal may also comprise horizontal and vertical sync information needed at the destination device to properly display the transmitted video signal. The horizontal and vertical sync signals may be transmitted over separate conductors than the video component signals. Alternatively, the sync signals may be combined with one or more of the video signal components and transmitted along with those components.
For RGB video, several different formats exist for conveying horizontal and vertical sync information. These include RGBHV, RGBS, RGsB, and RsGsBs. In RGBHV, the horizontal and vertical sync signals are each carried on separate conductors. Thus, five conductors are used: one for each of the red component, the green component, the blue component, the horizontal sync signal, and the vertical sync signal. In RGBS, the horizontal and vertical sync signals are combined into a composite sync signal and sent on a single conductor. In RGsB, the composite sync signal is combined with the green component. This combination is possible because the sync signals comprise pulses that are sent during a blanking interval, when no video signals are present. In RsGsBs, the composite sync signal is combined with each of the red, green and blue components. Prior art devices exist for converting from one format of RGB to another. To reduce cabling requirements, for transmission of RGB video over anything other than short distances, a format in which the sync signals are combined with one or more of the color component signals is commonly used.
Although twisted pair cables are convenient and economical for transmission of video signals, signal degradation (skew between video signal components and insertion loss) limits the distance over which satisfactory quality video signals can be transmitted via twisted pair cables. Video transmitter/receiver systems exist that amplify video signals transmitted over twisted-pair cables. In such systems, a transmitter amplifies the video source signal prior to being transmitted over twisted pair cable, and a receiver amplifies the received signal. These transmitter/receiver systems allow longer transmission distances over twisted-pair cable than are possible for unamplified signals. However, to prevent signal distortion, the amount of gain (amplification) supplied by the transmitter and receiver must be properly matched to the amount of insertion loss that occurs in the length of the twisted-pair cable over which the video signal is transmitted. If the gain applied it is too high, clipping will occur. If the gain is too low, low-level portions of the original input signal may be lost. Ideally the system gain should be flat across the frequency spectrum. High frequency loss results in smearing and loss of focus in the video.
There exists a need for a video transmission system that automatically compensates for signal losses that occur for video signals transmitted over appreciable distances via conductors, including twisted pair cables.